Polyglycerols are commonly prepared through thermal dehydration of glycerol, in which the polymerisation was carried out at atmospheric pressure and at an elevated temperature, which is about 230° C.-270° C. The process can be accomplished without the use of catalyst but the yield of polyglycerol is considerably low.
Therefore, various catalysts have been introduced to aid in the formation of polyglycerol and the most commonly used catalysts are alkaline catalysts such as sodium or potassium hydroxide, alkaline carbonates such as potassium carbonate with aluminium oxide and alkaline earth metal hydroxide such as calcium hydroxide.
Acidic catalysts were also used in the thermal dehydration of glycerol such as mixtures of sulphuric acid and triacetin, hypophosphorus acid with sodium hydroxide and acidic zeolite. In addition, clay such as hydrotalcite was also used to catalyse thermal dehydration of glycerol.
Polyglycerol formation was also reported with either solketal, glycidol or glycerol carbonate as the reactants when reacted with hydrotalcite at elevated temperatures. In addition, glycidol, glycerol carbonate and solketal were polymerised using the fluoride salts of rubidium, caesium and potassium into polyglycerol. Both linear and cyclic polyglycerols were reported as products of reaction between glycidol, glycerol carbonate and solketal with β-zeolites as catalysts.
A process to produce polyglycerol, which comprised reacting glycerol, diglycerol or higher polyglycerol with epichlorohydrin at 90° C. to 170° C. to produce a crude chlorohydrin/ether mixture, followed by adding an amount of strong base at least substantially equivalent to the organically bound chlorine content of the chlorohydrin/ether mixture, and desalting the mixture and recovering the glycerol, diglycerol and higher polyglycerol fractions has also been disclosed in the prior art.
Allyl alcohol is another route in preparing polyglycerols. The process involved epoxidation of the allyl alcohol, in which glycidol would be formed and then followed by polymerisation of the glycidol. This was proven as another effective method to prepare polyglycerol.
Despite the fact that the background art in preparing polyglycerol is crowded and diverse, it is evident that the synthesis of polyglycerol and diglycerol from glycerol has several drawbacks. One of the drawbacks is the use of high purity compounds such as glycerol, epiclorohydrin, glycidol, glycerol carbonate and solketal as the starting material in the preparation of polyglycerol. These chemical compounds are expensive and their cost makes up the bulk of the production cost of polyglycerol.
Another drawback of the prior arts is the fact that most of the prior arts needed catalysts that were introduced to the reactants at certain point of the production process. The introduction of catalyst to the reactants also increases the production cost of polyglycerol.
Therefore, it is an objective of this invention to provide a process that uses feedstock of lower purity, which contains a suitable catalyst for the reaction. This invention would provide a process to produce polyglycerol with lower production cost.